Diagnostic devices are used for a number of purposes in medicine, research, and other industries. For example in medicine, a diagnostic device may be used to measure the concentration of a particular substance (analyte) in a blood or urine specimen. It is important to ensure that the diagnostic device operates properly and that the test results returned are correct. In particular, it is important to ensure that the device is not operating with a systematic error that can corrupt a large portion of results produced.
When a sample is tested for the analyte, the instrument will return a test result, which may differ from the physically correct value. It is typically the case that test results provided by measuring devices have an inherent imprecision, that is, a predetermined range or margin of error. A test result may be deemed to be acceptable if the difference between the device's reported result and the correct value is within the inherent imprecision.
Since during use there often is no way to know the correct value of the specimens submitted for testing, the equipment may be periodically tested with reference samples to detect incorrectly reported results and systematic errors. In order to test the equipment, an operator may test one or more reference samples for which the correct result is known. Whereas a patient specimen may only be stable for a number of hours or days after collection, a reference sample may be a synthetic sample designed to be stable and testable for a much longer period of time, such as a number of months or years.
Once the testing values for the reference samples are obtained, they may be verified against a set of predetermined Quality Control (“QC”) criteria. Conducting this procedure, whether one or more samples are tested, may be referred to as a QC event. When more than one sample is tested, one sample with a corresponding normal value, one with an abnormally high value and one with an abnormally low may be tested to ensure that equipment returns correct results across the entire scale of results. Furthermore, a number of samples with the same value may be tested to ensure that the equipment consistently returns the same results.
If the results meet the QC criteria, the equipment is determined to be returning good results and accordingly not subject to any systematic errors, and it can be used to test further patient specimens. If the results do not meet the prescribed criteria, the equipment is likely to have started malfunctioning at some point before or during the QC event. The malfunctioning may have started after testing the last patient specimen, but before the QC event, in which case all the patient results will have been reported correctly. On the converse the error can have occurred at any point in time before the QC event, and all the results reported for the patient specimens tested following such failure may have been reported with an error greater than the acceptable margin of error.
If patient results obtained after a successful QC event are not released until the following QC event has been passed, the number of reported errors can be greatly reduced. The need for immediate release of test results often renders this option impractical, and another solution is therefore needed.
The operator can in general decrease the expected number of incorrectly reported patient results by increasing the number of QC events and by testing more reference samples at each QC event. However, increasing either of these increases cost and decreases the number of patient specimens that can be tested by the equipment during any period of time.